1. Field of the Invention
The present invention relates to a plastic lens having favorable optical characteristics and free of weld lines, which cause optical defects.
2. Description of the Related Art
In recent years, plastic lenses have become widely used in various types of imaging apparatuses consequent to the easy processing, reduced cost, and light weight of such lenses. Accordingly, many plastic lenses and manufacturing methods thereof have been proposed. Examples of such are recited in, for example, Japanese Patent Application Laid-Open Publication Nos. 2006-259561, 2006-272870, 2003-211500, and H7-117088. The plastic lenses recited in the patent documents above are manufactured by an injection molding method. This method involves fabricating a mold for forming the lens surface, injecting thermoplastic resin (hereinafter, simply “resin”) from a side gate of the cavity formed by the mold, and cooling/hardening of the lens.
However, when plastic lenses are formed by the injection molding method, the direction in which the resin flows inside the cavity is difficult to control. Consequently, melted resin flows to a portion forming the outer periphery of the lens before flowing to a portion forming a central portion of the lens, whereby a weld line, which is a vestige of a junction of the resin that flowed first in the formed lens and the resin that flowed thereafter, remains. This weld line easily arises when a plastic lens is formed having a central portion of a thickness that is relatively thin compared to the thickness at the periphery.
A remnant weld line in a lens invites deterioration of optical performance. When image sensors capable of realizing high resolution did not exist, even if a weld line remained, provided the weld line was slight, image degradation was negligible and was not a significant problem. However, accompanying technological advances of image sensors, high resolution can now be realized and even if a slight weld line remains in a lens, image degradation becomes obvious immediately, arising in a significant problem.